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Light weight kit car, 4 wheel drive.

Posted: Fri, 29 May 2009, 15:29
by Bluefang
Hey guys, i have been looking at building a light weight kit car, similar to a Lotus 7 style clubman. Will weigh in at about 250-300 as a rolling chassis. One thing i would like to do is electrify it :) I searched the forums for other kit car builds but couldnt find any :(

I am not a fan of gearboxs, diffs ect so i was planing on direct driving each wheel, or only 2 front, with seperate hub motors used in high power bike applications, about 5kw continues each if 4 wheeling it.

I have not found many that fit my criteria of top speed getting to 130km/h and having a pretty fast take off. http://www.goldenmotor.com/ Seems to be a pretty cheap place to import from, they have one motor that looks interesting, Brushless DC motor with 7kw@72V, will need to CV joint it to the wheels, but thats probably better anyway atm. Any one know about this motor/company or have sugestions on how to do it otherwise


Light weight kit car, 4 wheel drive.

Posted: Fri, 29 May 2009, 17:32
by fuzzy-hair-man
It's not a NotaFang (spelling?) that your looking at building is it?

Light weight kit car, 4 wheel drive.

Posted: Fri, 29 May 2009, 18:11
by Taffy
There are a number of EV lotus 7 replicas out there (3 that i know of in Aus), I am building 1 in melbourne, and there 2 in Sydney as well.
My blog is below but i have a long way to go!

With a 3.7Diff and using a Warp 9 or equivalent motor it is possible to just reach the 130 mark (in theory). If the motor can hold up the power 0-60 should be scary even with lead acids. 300kg is a fair aim for a rolling chassis, my aim is in that ball park with only electrical gear to fit.

Direct drive is hard on motors and difficult to control as you will need some kind of traction control system. This is because if 1 motor for some reasons spins faster/slower you dont want to be pushed into a turn at 100kph. It would be a case of monitor the wheels speeds and having some kind of power cut out if they get a % out sync - not sure how 1 would do this without building a controller yourself.

Its doable but abit harder then just plugging 1 big motor in a diff.
I know the RMIT F-SAE electric race team are building a twin motor set up that is direct drive. They use 5 controllers but last i heard how speed control each wheel was still an issue. only the last 2 control the power the ones above control the throttle settings, etc ensure that everything is in time.


Light weight kit car, 4 wheel drive.

Posted: Fri, 29 May 2009, 20:17
by Bluefang
hmm i was hoping that there would be some sort of wheel speed controller for this type of setup already which is why i have not listed one. pitty i dont have much programing knowledge to build a computer controlled system, A simple VR sensor on each drive wheel should be easy to setup with some extra physics behind it to give greater speed/power to outside wheels on a % basis of the steering input.

Your build is looking great :) are you building from plans or just going with what you want and hopeing the inspecters give you a nice big tick at the end. I might end up going with a single motor and diff seeing as it will be my first EV and its a tried and true method of getting some good results, and i ll look in on the multi wheel controller for another build.

Do u know if the other kit car guys have blogs/webpages on their builds?

Light weight kit car, 4 wheel drive.

Posted: Fri, 29 May 2009, 21:21
by Tritium_James
For multiple wheels you don't want to control wheel speed, that would give you a 'locked diff' during cornering. What you want is both wheels doing the same torque. This can be fairly easily achieved with a decent motor controller that can regulate motor (phase) current, as motor current is proportional to torque.

So you command both controllers to do the same motor current, with current proportional to throttle pedal position, and the speed issues during cornering take care of themselves automatically.

Light weight kit car, 4 wheel drive.

Posted: Fri, 29 May 2009, 21:47
by Taffy
I was meaning that as you turn you want slow the inside wheel/ speed up the outside wheel as you turn. Due to length of travel through a turn is further for the outside wheel.
But at speed in a straight line you dont want them spinning a different speeds and powering 1 side more then the other.

I have not seen another blog as such, some pics of the fnished cars going around so i am flying blind.

My chassis is based off the book by Ron Champion, How to build a sports car for less then $250 (pounds). But with alot of mods.

Light weight kit car, 4 wheel drive.

Posted: Fri, 29 May 2009, 22:51
by Tritium_James
That's exactly what I'm talking about. Both the fact that the outside wheel has to go faster (further) through a corner, and that you want them both doing the same thing in the straights. All of this happens automatically if you run both motors at the same phase current.

Light weight kit car, 4 wheel drive.

Posted: Fri, 29 May 2009, 23:09
by acmotor
TJ is on the money. Electrically with two motors the 'diff' is quite simple.

The electromotive differential ! In fact the outer wheel has more traction and can have more torque applied than the inner wheel.
This is done in some active stability control and traction control systems but in mechanical ways e.g. by applying brakes.

The individual electric drive (e.g. hub motors) offers the ultimate in control. Step 1 is just ask for the same torque and the revs sort themselves out as TJ says. Step 2 is make some upmarket performance decisions about where to put the power because you can ! Image

Light weight kit car, 4 wheel drive.

Posted: Sat, 30 May 2009, 00:15
by Tritium_James
Yes, the ultimate system looks at G forces, yaw rates, and steering angles to calculate the downforce and side loads on each wheel, then applies the appropriate amount of torque to each to give best possible acceleration and cornering performance.

All quite exciting, it's possible now, just worth $$$. For a non-electric version of this, see: http://www.nissan.com.au/webpages/model ... model.html

Light weight kit car, 4 wheel drive.

Posted: Sat, 30 May 2009, 17:02
by juk
Awesome posts James.

A simple accelerometer can achieve 90% of what you just mentioned combined with some appropriate algorithms. Steering angle is not required, as you're only looking at forces. thus you only need to compensate for forces as they happen. So a simple 3 axis accelerometer can do it all.

Basically you could use a playstation controller for your sensor, or an iPhone and you basically transfer torque opposite to the forces. It could also be used to optimise regenerative braking too.

Light weight kit car, 4 wheel drive.

Posted: Sun, 31 May 2009, 18:06
by Bluefang
Well with all that extra hassles that go along with dual motors i doubt i ll be building around that design. I am thinking atm that a simple mid seating spaceframe chassis(clubman with the seats moved forward) with a single motor similar to warp 9 mounted parallel to the rear axle with a chain/belt drive joining them to save on the weight of a diff and drive shaft, or a much shorter motor pluged directly into a diff. Now all i have to do is find a free design of something that looks good :). I finaly found the nota-fang Fuzzy hair man mentioned not quite my taste

Light weight kit car, 4 wheel drive.

Posted: Sun, 31 May 2009, 23:57
by Taffy
My Suggestions would be to take the assemble from a front wheel car and put that in the back. Lock the steering straight and you have an independent set up, a small diff and a gearbox set up in 1 go. Or no gearbox. Maybe take an MX5 one? or a Barina.
There are some designs done around making a mid engined clubbie which could be copied for that kind of set up.
IMO opinion go down the track someone else has, it will save you alot of time. I will take a look and see what i can find.


Light weight kit car, 4 wheel drive.

Posted: Mon, 01 Jun 2009, 16:58
by Benonymous
If you're building the Ron Champion chassis, be aware that it will not achieve the 4000kg/in torsional rigidity required for registration. There is a PDF floating around somewhere with additional tubes that must be added to the chassis to get it up to spec. If you really want to go 4WD why not start with a Subaru Impreza as a donor? The dual motor setup has been done on these using two rear diffs I think.

Light weight kit car, 4 wheel drive.

Posted: Mon, 01 Jun 2009, 19:03
by Bluefang
I was not specificaly looking to go 4 wheel setup, I was more looking at using hub/inwheel motors. Seeing as they are still fairly low powered i was thinking that 4 motorbike sized ones would get the desired results of a fairly fast car in a light weight kit car. But if there is nothing really to control them all i might just stick to the tried and true single motor with a rear diff/chain drive until a controller becomes commercially available. While a Subaru would be ok, i dont like the look of it and there too heavy.

I think i ll just take my time and build a chassis around a EV car design, and yeah i had heard about the modifiactions needed to the Ron Champions chassis, i ll be speaking to a engineer before i build anything and the design is going to be a fairly long way from the Locost design.

Light weight kit car, 4 wheel drive.

Posted: Tue, 02 Jun 2009, 18:43
by fuzzy-hair-man
Bluefang wrote: I finaly found the nota-fang Fuzzy hair man mentioned not quite my taste
I was just guessing based off your username, guess I guessed wrong... Image they're not my taste either, just I'd heard of them. Image